1. Field of the Invention
The present invention relates generally to a method and apparatus for treating surfaces, such as for the removal of coatings and contamination from that surface. In particular, this invention relates to a method and apparatus utilizing a laser-guided gas-embedded pinchlamp device for surface treatment. The present invention is an improvement over prior flashlamps, used, for example, for the removal of coatings.
2. Description of Related Art
The transfer of heat to surfaces to facilitate surface treatment and/or the removal of coatings is well known. In recent decades, this transfer has been completed through the use of propane, kerosene, and acetylene torches, as well as electrical heat guns, quartz-halogen lamps, and high pressure concentrated arc lamps employing noble gases. These approaches to coating divestment from a surface deposit energy in the coatings at typical heating rates ranging from 100 to 1000 W/cm.sup.2. However, at such low heating rates, it usually requires many seconds of residence time to destroy a coating's adhesion or integrity, facilitating the convenient removal of any residual debris such as ash. A disadvantage to such approaches is that all but the most durable substrate materials employed in structures, for example steel or concrete, suffer unacceptable thermal damage. A further disadvantage to such low-temperature pyrolysis processes, is that these treatments frequently expel large quantities of highly toxic vapors and smoke.
Since the early 1960s, the ruby laser has been utilized to create higher heating rates than those above, rates approaching 10.sup.4 to 10.sup.6 W/cm.sup.2. At these heating rates, the sublimation, ablation, spallation, dissociation, or pyrolysis wavefront eating into the coating may overtake the thermal conduction wave that heats the substrate, and therefore, very little net substrate heating occurs.
In the 1970s, Nd:YAG laser systems in the 1-10 Watt average power range were employed in a number of specialized, high-valued applications for coating removal. Some twenty years ago, the customer was willing to pay the coating removal costs of $100/ft.sup.2 or more, and was amenable to the manual control of these techniques. Later, the photon generator of choice was the CO.sub.2 laser. As the CO.sub.2 laser was economical and successfully scaled to kW power ranges, it began to be used in automated paint stripping situations. Gas lasers, such as the CO.sub.2 laser, tended to be rather large and bulky, a disadvantage in such paint removal applications. As a second disadvantage, the infrared wavelength generated by a CO.sub.2 laser did not lend itself to coating color discrimination and fiberoptic beam delivery which compounded the size problem.
By the mid 1970s, optical radiation from a pair of 9 mm-bore, 12 inch long linear xenon flashlamps driven by an available plasma containment power supply at the University of California, San Diego was utilized to strip paint from an historic, oilcloth surface. The flashlamp-charred, over-paint residue left on the surface after coating removal, was manually removed from the set cushions with a mild alcohol solution.
By the mid-1980's, flashlamp technology had evolved such that efforts were underway to attempt the construction of a commercially viable flashlamp paint stripping system. This system was later abandoned. Problems arising from this effort revealed that the ash residue left by the treatment still had to be removed by hand. Further, the various designs of the flashlamps, reflectors and airflow systems were such that they required the frequent cleaning of hard to reach, difficult surfaces. Their performance also began to significantly decline after approximately 10,000 flashlamp pulses, thereby necessitating frequent bulb replacement. Finally, in order to achieve a commercially desired stripping rate, the system had to be operated at a higher than anticipated pulse repetition rate, approximately 6 Hz, where such use caused some substrate materials to overheat.
In response to the deficiencies of flashlamp technology, the next generation of high-power radiation resources evolved, that being the pinchlamp. The gas-embedded pinchlamp differs from the flashlamp in that a beam from a very small laser is used to initiate the plasma `pinch` within the enclosed pressurized gas, wherein the hot radiating plasma created is prevented from contacting the containment vessel, typically a transparent quartz envelope. The pinchlamp's unique design has been used for nuclear waste remediation and chemical weapons neutralization.
An example of this technology is Pumping a Photolytic Laser Utilizing a Plasma Pinch, U.S. Pat. No. 4,450,568 to Asmus. This particular pinchlamp device requires a laser medium within the quartz tube, an element that is not needed in the present invention. Currently, experimental pinchlamps typically are some 10-100 times more intense than the prior art flashlamps. The gas-embedded pinch device typically comprises a high pressure argon gas contained within a large quartz tube. A beam from a very small laser is directed down the axis of the tube and creates a straight and narrow, generally 5 mm in width, conductive path for a high power electrical discharge. The electrical discharge heats the dense argon channel to a very high temperature and thereafter, large amounts of radiation are produced without the plasma coming into physical contact with the quartz wall, which would destroy the wall. Typically, laser-guided gas-embedded pinchlamps operate on a single shot basis. No significant attempt has been undertaken to fabricate a device having a repetition rate of between 5-10 Hz in this particular technology, as is accomplished by the present invention.
Liquid-jet pinchlamps, like the gas-embedded pinch, have been utilized for various purposes. The liquid pinch comprises the shooting of a thin stream, generally 100 um, of liquid decane into a vacuum chamber. As the decane traverses the chamber, a small amount of the liquid evaporates, creating a tenuous vapor cloud around the jet. Then a high electrical potential is applied from one end of the jet to the other and a small electrical current flows through the cloud. The UV radiation from the cloud heats the liquid to the point of electrical conduction. Then a very large electrical current flows through the newly created conducting liquid, and heats it to very high temperatures so that high-intensity radiation is produced.
Unlike the liquid jet pinchlamp, the laser-guided gas-embedded pinchlamp is free of the vacuum pumping element of the liquid-jet pinch device. See, for example, U.S. Pat. No. 4,889,605 to Asmus. Yet, it does depend on a small laser for control. The laser-guided gas-embedded pinch device is scaleable to a range of about 500-1000 kW average power, whereas the liquid-jet pinch is limited to about 40 kW average power. The higher output of this device indicates that it is vastly more effective than flashlamp light in coupling to surfaces and inducing profound effects. With the pinchlamp's small effective source size, it may be imaged so as to produce these effects at a much greater standoff distance than the flashlamp (10-20 cm). The ability to operate in open-air environments makes this far more versatile technology than liquid-jet pinch devices which are limited to operating in a vacuum environment.
Current aircraft paint strip rates using chemicals are up to 8 to 12 square feet per minute, at a thickness at approximately 3 mils overall. This excludes composite material surfaces which need to be masked during chemical application and then sanded separately. In order to make an optical system economically feasible, this strip rate of 8 to 12 square feet per minute is unacceptable in some instances. A device that can paint strip at approximately 15 to 30 square feet per minute, including the stripping of composite substrates, is required for advanced high-performance military aircraft. Further, the removal of paint and coatings must be performed without damage to the substrate, and it is most desirable that depth of removal be controlled. Additionally, a deployable system if used, must be robust and reliable, and must further be operationally cost competitive with available systems.
Thus, it can be seen that there is a need for the present invention, an improvement over the prior art removal systems, including plastic media blasting, wheat blasting, and more recently, the flashlamp. It is the provision such an apparatus that the present invention is primarily directed.